1) Field of the Invention
The present invention relates to a digital amplifier for an audio system. More particularly, this invention relates to a digital amplifier that adopts a pulse width modulation (PWM) system.
2) Description of the Related Art
Battery-operated apparatuses having built-in speakers, such as notebook size personal computers, portable compact disk (CD) players, digital versatile disc (DVD) players, and car audio systems are in common use. In these apparatuses, not only sound quality but also miniaturization and low power consumption are demanded. Due to such background, an attention is paid to digital amplifiers. In a digital amplifier, of these digital amplifiers, that adopts a PWM system used for a pre-amplifier for an audio system, all sections from an input to an output are formed with digital circuits, and therefore all sound signals can be digitally processed. The PWM system can convert a voltage amplitude of sound information into a digital pulse width to thereby directly drive the speakers, and therefore an analog process is not necessary. Accordingly, a compact size amplifier that requires low power and less heating value can be realized.
However, when the PWM system digital amplifier and the speakers are connected by bridge-tied load (BTL) that drives a load using two outputs of a positive polarity and a negative polarity, there arises a problem that a pop sound is produced when the output of the digital amplifier abruptly changes.
In order to improve the problem, in the conventional technology, a level of saw-tooth waveform obtained by converting an oscillation output that has a higher frequency than a frequency of a carrier signal supplied to a pulse width modulator and is synchronized with the carrier signal, is compared with a level of an output of a time constant circuit to which a power supply voltage is applied. The output obtained through the level comparison and the output of the pulse width modulator are supplied to an exclusive-OR circuit and therefore the two outputs have the same phase at the times at which the power is turned on and turned off so as to suppress the production of the pop sound (see Japanese Patent Application Laid-Open No. 06-196940 (1994)).
In the conventional technology, however, when the phase of the output of the exclusive-OR circuit is in a transition state, a sound signal output to the load is disadvantageously distorted. Suppression of the distortion causes a problem to occur such that muting should be applied to the output of the exclusive-OR circuit in a specified period in which the phase of the output of the exclusive-OR circuit is in the transition state.
There is another problem that an integrated circuit and the time constant circuit are used and therefore the process cannot be executed only by using digital signals.